System and method for independently operating multiple neurostimulation channels

ABSTRACT

A multi-channel neurostimulation system comprises a plurality of electrical terminals configured for being respectively coupled to a plurality of electrodes, stimulation output circuitry including electrical source circuitry of the same polarity configured for generating a plurality of pulsed electrical waveforms in a plurality of timing channels, and control circuitry configured for instructing the stimulation output circuitry to serially couple the electrical source circuitry to different sets of the electrodes when pulses of the respective pulsed electrical waveforms do not temporally overlap each other, and for instructing the stimulation output circuitry to couple the electrical source circuitry to a union of the different electrode sets when pulses of the respective pulsed electrical waveforms temporally overlap each other.

RELATED APPLICATION

The present application claims the benefit under 35 U.S.C. §119 to U.S.provisional patent application Ser. No. 61/291,058, filed Dec. 30, 2009.The foregoing application is hereby incorporated by reference into thepresent application in its entirety.

FIELD OF THE INVENTION

The present invention relates to tissue stimulation systems, and moreparticularly, to a system and method for operating multipleneurostimulation channels.

BACKGROUND OF THE INVENTION

Implantable neurostimulation systems have proven therapeutic in a widevariety of diseases and disorders. Pacemakers and Implantable CardiacDefibrillators (ICDs) have proven highly effective in the treatment of anumber of cardiac conditions (e.g., arrhythmias). Spinal CordStimulation (SCS) systems have long been accepted as a therapeuticmodality for the treatment of chronic pain syndromes, and theapplication of tissue stimulation has begun to expand to additionalapplications such as angina pectoralis and incontinence.

More pertinent to the present inventions described herein, Deep BrainStimulation (DBS) has been applied therapeutically for well over adecade for the treatment of neurological disorders. DBS and otherrelated procedures involving implantation of electrical stimulationleads within the brain of a patient are increasingly used to treatdisorders, such as Parkinson's disease, essential tremor, seizuredisorders, obesity, depression, obsessive-compulsive disorder,Tourette's syndrome dystonia, and other debilitating diseases viaelectrical stimulation of one or more target sites, including theventrolateral thalamus, internal segment of globus pallidus, substantianigra pars reticulate, subthalamic nucleus (STN), or external segment ofglobus pallidus.

DBS has become a prominent treatment option for many disorders, becauseit is a safe, reversible alternative to lesioning. For example, DBS isthe most frequently performed surgical disorder for the treatment ofadvanced Parkinson's Disease. There have been approximately 30,000patients world-wide that have undergone DBS surgery. Consequently, thereis a large population of patients who will benefit from advances in DBStreatment options. Further details discussing the treatment of diseasesusing DBS are disclosed in U.S. Pat. Nos. 6,845,267 and 6,950,707, whichare expressly incorporated herein by reference.

Implantable neurostimulation systems typically include one or moreelectrode carrying stimulation leads, which are implanted at the desiredstimulation site, and a neurostimulator (e.g., an implantable pulsegenerator (IPG)) implanted remotely from the stimulation site, butcoupled either directly to the stimulation lead(s) or indirectly to thestimulation lead(s) via a lead extension. The neurostimulation systemmay further comprise an external control device to remotely instruct theneurostimulator to generate electrical stimulation pulses in accordancewith selected stimulation parameters.

Electrical stimulation energy may be delivered from the neurostimulatorto the electrodes in the form of a pulsed electrical waveform. Thus,stimulation energy may be controllably delivered to the electrodes tostimulate neural tissue. The combination of electrodes used to deliverelectrical pulses to the targeted tissue constitutes an electrodecombination, with the electrodes capable of being selectively programmedto act as anodes (positive), cathodes (negative), or left off (zero). Inother words, an electrode combination represents the polarity beingpositive, negative, or zero. Other parameters that may be controlled orvaried include the amplitude, duration, and frequency of the electricalpulses provided through the electrode array. Each electrode combination,along with the electrical pulse parameters, can be referred to as a“stimulation parameter set.”

With some neurostimulation systems, and in particular, those withindependently controlled current or voltage sources, the distribution ofthe current to the electrodes (including the case of theneurostimulator, which may act as an electrode) may be varied such thatthe current is supplied via numerous different electrode configurations.In different configurations, the electrodes may provide current orvoltage in different relative percentages of positive and negativecurrent or voltage to create different electrical current distributions(i.e., fractionalized electrode configurations).

In the context of DBS, a multitude of brain regions may need to beelectrically stimulated in order to treat one or more ailmentsassociated with these brain regions. To this end, multiple stimulationleads are typically implanted adjacent the multiple brain regions. Inparticular, multiple burr holes are cut through the patient's cranium asnot to damage the brain tissue below, a large stereotactic targetingapparatus is mounted to the patient's cranium, and a cannula isscrupulously positioned through each burr hole one at a time towardseach target site in the brain. Microelectrode recordings may typicallybe made to determine if each trajectory passes through the desired partof the brain, and if so, the stimulation leads are then introducedthrough the cannula, through the burr holes, and along the trajectoriesinto the parenchyma of the brain, such that the electrodes located onthe lead are strategically placed at the target sites in the brain ofthe patient.

Stimulation of multiple brain structures (i.e., different functionalregions of the brain) with different sets of stimulation parameters hasbeen shown to be useful. For example, stimulation of thePedunculopontine (PPN) and Subthalamic Nuclei (STN) at differentfrequencies has been shown to be beneficial (see Alessandro Stefani, etal. “Bilateral Deep Brain Stimulation of the Pedunculopontine andSubthalamic Nuclei in Severe Parkinson's Disease,” Brain (2007); 1301596-1607). In another DBS example, one frequency is used to optimizetreatment of tremor and rigidity, while another frequency is used totreat bradykinesia (see U.S. Pat. No. 7,353,064).

Thus, if the same set of stimulation parameters is used to stimulate thedifferent brain structures, either (1) one brain structure may receiveoptimal therapy and the other brain structure may receive poor therapy,or, (2) both brain structures may receive mediocre therapy. Thus, tomaximize the therapeutic effects of DBS, each brain structure mayrequire different sets of stimulation parameters (i.e. differentamplitudes, different durations, and/or frequencies).

One way that prior art DBS techniques attempt to stimulate several brainstructures using different stimulation parameters is to implant multipleleads adjacent the different regions of the brain, and to quickly cyclethe stimulation through the brain structures with the differentstimulation parameters. In some applications, such as the treatment ofchronic pain, this effect may be unnoticeable; however, the brain is acomplex system of rapidly transmitting electric signals, and the effectof rapid cycling may produce a “helicopter effect” that may undesirablyresult in ineffective treatment and/or side-effects such as seizures.

Another way that prior art DBS techniques attempt to stimulate severalbrain structures using different stimulation parameters is to connectthe multiple leads to multiple neurostimulators respectively programmedwith different stimulation parameters. However, this increases the costof the procedure, increases the length of the procedure, and increasesthe risks associated with the surgery.

Another approach is to use multiple timing channels when applyingelectrical stimulation to different brain structures. Each timingchannel identifies the combination of electrodes used to deliverelectrical pulses to the targeted tissue, as well as the characteristicsof the current (pulse amplitude, pulse duration, pulse frequency, etc.)flowing through the electrodes. Because prior art neurostimulationsystems are incapable of simultaneously controlling the generation ofelectrical pulses (e.g., either because they comprise only one anodicelectrical source and one cathodic electrical source or otherwisebecause one or more stimulation parameters used to define one electricalpulse may be overwritten with one or more stimulation parameters used todefine a subsequent overlapping electrical pulse), the use of multipletiming channels can often lead to issues due to the potential of anoverlap in electrical pulses between two or more timing channels. Theseneurostimulation systems may time-multiplex the pulsed electricalwaveforms generated in each of the multiple channels to preventelectrical pulses in the respective channels from overlapping eachother.

For example, with reference to FIG. 1, one prior art neurostimulationcontroller 1 that is capable of controlling output stimulation circuitry2 to output up to four pulsed electrical waveforms respectively overfour timing channels in accordance with four stimulation parameter sets.The output stimulation circuitry 2 includes a single anodic currentsource 3 a and an associated decoder 4 a (or bank of decoders), and asingle cathodic current source 3 b and an associated decoder 4 b (orbank of decoders). The decoder 4 a is configured for decoding a digitalcode defining an anodic electrode combination (i.e., the active anodicelectrodes) and amplitude values for the anodic electrode combination,and the decoder 4 b is configured for decoding a digital code defining acathodic electrode combination (i.e., the active cathodic electrodes)and amplitude values for the cathodic electrode combination.

The neurostimulation controller 1 comprises a number of registers 5 (inthis case, four registers 1-4), each of which digitally stores certainparameters of one of the four stimulation parameter sets, and inparticular, the electrode combination (i.e., the active electrodes) andamplitude and polarity (cathode or anode) of each of the active ones ofthe electrode combination. The neurostimulation controller 1 furthercomprises a number of timers 6 (in this case, four timers 1-4), each ofwhich controls the pulse duration and frequency of one of the fourstimulation parameter sets by outputting a high/low signal.

The neurostimulation controller 1 further comprises amultiplexor/selector 7 that outputs the digital contents (electrodecombination, amplitude, and polarity) of a selected one of the registers5 to the decoders 3 of the stimulation output circuitry 1 when thesignal output by the respective timer 6 to the multiplexor/selector 7 ishigh (i.e., a logical 1 on one of the timers 6 gates the associatedregister 5 to the output of the multiplexor/selector 7). The stimulationoutput circuitry 2 then outputs an anodic electrical pulse and acathodic electrical pulse in accordance with the electrode combination,amplitude, and polarity defined by the digital contents of therespective register 5 and decoded by the decoders 3, and the pulse widthand frequency defined by the respective timer 6.

The neurostimulation controller 1 further comprises an arbitrator 8 forserially selecting the timing channels in which anodic and cathodicpulses will be output by the stimulation output circuitry 2 by seriallyturning on the timers 6, and thus, serially outputting the digitalcontents of the respective register 5 to the stimulation outputcircuitry 5. The arbitrator 8 selects the timing channels in a mannerthat prevents overlap of electrical pulses between the channels to avoidthe aforementioned problems associated with attempting to generateoverlapping pulses using single-source output circuitry. Notably, forthe specific architecture illustrated in FIG. 1, preventing overlap ofelectrical pulses will ensure that information of a current electricalpulse (i.e., the digital contents obtained from one of the registers 5)stored within the decoders 3 of the stimulation output circuitry 2 isnot overwritten with information of an overlapping electrical pulse(i.e., the digital contents obtained from another of the registers 5)when subsequently stored in the decoders 3 of the stimulation outputcircuitry 2.

If the frequencies of two pulsed electrical waveforms are the same or aharmonic of the other, the electrical pulses can be easily spaced intime within the respective channels, such that they do not coincide, asillustrated by the pulsed electrical waveforms in FIG. 2. For purposesof simplicity, only the anodic portion of the pulsed electricalwaveforms is shown. When the frequencies of two pulsed electricalwaveforms are not the same or otherwise not a harmonic of each other,the pulses of the pulsed electrical waveform with the faster frequencywill “walk” over the pulses in the other pulsed electrical waveform, andtherefore, there will be occasions when the pulses in the respectivechannels will need to be simultaneously generated, as illustrated inFIG. 3.

However, when there is only one source for each polarity, as shown inFIG. 1, or at least, when there are one or more non-dedicated sources(i.e., a source that can be shared by multiple electrodes), twoelectrical pulses of the same polarity cannot be generatedsimultaneously due to the potential of digitally overwriting theelectrode combination information of the first electrical pulse with theelectrode combination information associated with the second electricalpulse. Thus, even though multiple pulsed electrical waveforms can begenerated in multiple channels, they must all have related frequenciesto maintain a constant period, unless at least one pulsed electricalwaveform is modified.

For example, in one embodiment, the arbitrator 8 uses a method known asthe “token” method to prevent overlap of stimulation pulses betweenchannels by modifying one or more of the pulsed electrical waveforms.This method allows an electrical pulse to be transmitted in the timingchannel with the “token,” while the other timing channels wait theirturn. Then, the “token” is passed to the next timing channel. However,if the channels overlap, such that they need the “token” at the sametime, transmission of an electrical pulse within the second channel mustwait until the end of the transmission of the electrical pulse in thefirst timing channel. The arbitrator 8 accomplishes this by putting thetimer 6 associated with the subsequent electrical pulse on hold whilethe output of the multiplexor/selector 7 is in use.

The “token” method may best be understood with reference to FIG. 4. Asthere shown, a first pulsed electrical waveform 9 a having a firstfrequency is transmitted within timing channel A, and a second pulsedelectrical waveform 9 b having a second frequency is desired to betransmitted within timing channel B. Because timing channel A has the“token,” the pulses of the second pulsed electrical waveform 9 b thatare to be transmitted in timing channel B must be “bumped” each timethey overlap with the pulses of the first pulsed electrical waveform 9a. As can be seen in the bumped pulsed electrical waveform 9 c, when apulse is bumped (shown by the horizontal arrows), the next pulse relieson the new (bumped) pulse for timing. Thus, the next pulse is “doublebumped”: once when the previous pulse is bumped and a second time whenit overlaps a pulse of the pulsed electrical waveform 9 a transmitted inthe timing channel A. As a result, the frequency of the pulses in thesecond pulsed electrical waveform 9 b is forced (i.e., locked) into thefrequency for the first pulsed electrical waveform 9 a, resulting in apulsed electrical waveform 9 d that has a frequency twice as small asthe desired frequency.

One adverse result of using the token method is that the frequency ofthe electrical pulses transmitted in the second timing channel gets“locked” to (i.e. matches) the frequency of the electrical pulsestransmitted in the first timing channel; alternatively, one can getgalloping or clumping of electrical pulses. Therefore, when theoccurrence of electrical pulses is pushed out in time, stimulationtherapy may become ineffective or even harmful for tissue regions, suchas brain structures to be stimulated in DBS applications, that requirestimulation at specific, regular frequencies (See Birno M J, Cooper S E,Rezai A R, Grill W M, Pulse-to-Pulse Changes in the Frequency of DeepBrain Stimulation Affect Tremor and Modeled Neuronal Activity, J.Neurophysiology, 2007 September; 98(3): 1675-84.

There, thus, remains a need to provide an improved technique forindependently operating multiple stimulation channels in aneurostimulation system where at least one electrical source in theneurostimulation system is shared by a plurality of electrodes.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present inventions, a multi-channelneurostimulation system is provided. The neurostimulation systemcomprises a plurality of electrical terminals configured for beingrespectively coupled to a plurality of electrodes, and stimulationoutput circuitry including electrical source circuitry of the samepolarity configured for generating a plurality of pulsed electricalwaveforms in a plurality of timing channels. The pulsed electricalwaveforms may have different pulse frequencies. In one embodiment, thestimulation output circuitry includes at least one switch bank coupledbetween the electrical source circuitry and the electrical terminals.

The neurostimulation system further comprises control circuitryconfigured for instructing the stimulation output circuitry to seriallycouple the electrical source circuitry to different sets of theelectrodes when pulses of the respective pulsed electrical waveforms donot temporally overlap each other, and for instructing the stimulationoutput circuitry to couple the electrical source circuitry to a union ofthe different electrode sets when pulses of the respective pulsedelectrical waveforms temporally overlap each other.

The neurostimulation system may further comprise a housing containingthe plurality of electrical terminals, stimulation output circuitry, andcontrol circuitry. Alternatively, some of the components of theneurostimulation system may be contained in separate housings. In oneembodiment, the electrical source circuitry comprises a current source.In this case, electrical source circuitry may comprise a plurality ofcurrent branches, and the control circuitry may be configured forselecting a current magnitude for each electrode in the union of thedifferent electrode sets by assigning one or more of the currentbranches to the respective electrode.

In another embodiment, the pulsed electrical waveforms are defined by arespective plurality of stimulation parameter sets, in which case, thecontrol circuitry is configured for obtaining a digital representationof an electrode set from each of the stimulation parameter sets,combining the digital representations together to create a union of thedigital representations, and outputting the digital representation unionto the stimulation circuitry, and the stimulation output circuitry isconfigured for coupling the electrical source circuitry to the union ofthe different electrode sets in accordance with the digitalrepresentation union. Each of the digital representations may comprise adigital representation of current amplitude values for the respectiveelectrode set, in which case, the control circuitry may be configuredfor instructing the stimulation output circuitry to supply electricalcurrent from the electrical source circuitry to the different sets ofthe electrodes or the union of the different electrode sets inaccordance with the current amplitude values.

In still another embodiment, the control circuitry includes a pluralityof registers configured for storing digital representations of theelectrode sets, and a plurality of timers configuring for outputtingphase enabling signals in accordance with a pulse duration and frequencyof the pulsed electrical waveforms. The phase enabling signal output byeach of the timers may, e.g., be high when the pulse of the respectivepulsed electrical waveform is active. The control circuitry may furtherinclude a plurality of AND gates, each of which has an input coupled toan output of a respective one of the registers and an input coupled toan output of a respective one of the timers, and an OR gate havinginputs coupled to respective outputs of the AND gates, and an coupled toan input of the stimulation output circuitry.

In accordance with a second aspect of the present inventions, anothermulti-channel neurostimulation system is provided. The neurostimulationsystem comprises a plurality of electrical terminals configured forbeing respectively coupled to a plurality of electrodes, a plurality ofregisters configured for storing a respective plurality of digitalrepresentations of different sets of the electrodes, and a plurality oftimers configuring for outputting phase enabling signals in accordancewith a pulse duration and pulse frequency of a respective plurality ofpulsed electrical waveforms. The pulsed electrical waveforms may havedifferent pulse frequencies. In one embodiment, the timers are operatedindependently of each other.

The neurostimulation system further comprises a plurality of AND gates,each of which has an input coupled to an output of a respective one ofthe registers and an input coupled to an output of a respective one ofthe timers, and an OR gate having inputs coupled to respective outputsof the AND gates.

The neurostimulation system further comprises stimulation outputcircuitry having an input coupled to an output of the OR gate. Thestimulation output circuitry includes electrical source circuitry of thesame polarity programmable to selectively couple to the electrodes viathe electrical terminals based on the output of the OR gate. Theneurostimulation system may further comprise a housing containing theplurality of electrical terminals, the plurality of registers, theplurality of timers, the plurality of AND gates, the OR gate, and thestimulation output circuitry. Alternatively, some of the components ofthe neurostimulation system may be contained in separate housings.

In one embodiment, the electrical source circuitry comprises a currentsource. In this case, each of the digital representations may comprise adigital representation of current amplitude values for the respectiveelectrode set, and the electrical source circuitry may be programmableto supply current to the electrodes based on the output of the OR gate.The electrical source circuitry may comprise a plurality of currentbranches, in which case, the neurostimulation system may furthercomprise a branch distribution circuit configured for assigning one ormore of the current branches to each of the electrodes based on theoutput of the OR gate. Each current branch may comprise a switch bankcoupled to the electrodes, and a decoder coupled to the respectiveswitch bank, wherein the branch distribution circuit is configured forsupplying a digital code to each of the decoders, and the digital codedefines one of the electrodes to be coupled to the electrical sourcecircuitry via the respective switch bank.

Other and further aspects and features of the invention will be evidentfrom reading the following detailed description of the preferredembodiments, which are intended to illustrate, not limit, the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings illustrate the design and utility of preferred embodimentsof the present invention, in which similar elements are referred to bycommon reference numerals. In order to better appreciate how theabove-recited and other advantages and objects of the present inventionsare obtained, a more particular description of the present inventionsbriefly described above will be rendered by reference to specificembodiments thereof, which are illustrated in the accompanying drawings.Understanding that these drawings depict only typical embodiments of theinvention and are not therefore to be considered limiting of its scope,the invention will be described and explained with additionalspecificity and detail through the use of the accompanying drawings inwhich:

FIG. 1 is a block diagram of prior art control circuitry for preventingoverlap between pulses of electrical waveforms programmed in multipletiming channels;

FIG. 2 is a plot illustrating two pulsed electrical waveforms generatedin two timing channels by a prior art system, wherein the waveforms havethe same pulse frequency, such that pulses of the waveforms do notoverlap;

FIG. 3 is a plot illustrating two pulsed electrical waveforms generatedin two timing channels by a prior art system, wherein the waveforms havedifferent pulse frequencies, such that pulses of the waveforms overlap;

FIG. 4 is timing diagram illustrating a prior art technique forpreventing the overlap between pulses of electrical pulsed waveformsprogrammed in multiple timing channels;

FIG. 5 is a plan view of an embodiment of a deep brain stimulation (DBS)system arranged in accordance with the present inventions;

FIG. 6 is a profile view of an implantable pulse generator (IPG) andpercutaneous leads used in the DBS system of FIG. 5;

FIG. 7 is a timing diagram illustrating four exemplary pulsed electricalwaveforms generated in four respective timing channels by the IPG ofFIG. 6;

FIG. 8 is a plan view of the DBS system of FIG. 5 in use with a patient;

FIG. 9 is a block diagram of the internal components of the IPG of FIG.6;

FIG. 10 is a block diagram of stimulation output circuitry contained inthe IPG of FIG. 6;

FIG. 11 is a block diagram of control circuitry contained in the IPG fordefining pulsed electrical waveforms in a manner that allows overlap ofpulses within respective timing channels;

FIGS. 12 a-12 d illustrate digital representations of active electrodes,electrode polarity, and amplitude values for timing channels 1-4 storedin registers of the control circuitry of FIG. 11; and

FIG. 13 illustrates a union of the digital representations illustratedin FIGS. 12 a and 12 b for timing channels 1 and 2.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The description that follows relates to a deep brain stimulation (DBS)system. However, it is to be understood that the while the inventionlends itself well to applications in DBS, the invention, in its broadestaspects, may not be so limited. Rather, the invention may be used withany type of implantable electrical circuitry used to stimulate tissue.For example, the present invention may be used as part of a pacemaker, adefibrillator, a cochlear stimulator, a retinal stimulator, a stimulatorconfigured to produce coordinated limb movement, a cortical stimulator,a spinal cord stimulator, peripheral nerve stimulator, microstimulator,or in any other neural stimulator configured to treat urinaryincontinence, sleep apnea, shoulder sublaxation, headache, etc.

Turning first to FIG. 5, an exemplary DBS neurostimulation system 10generally includes one or more (in this case, two) implantablestimulation leads 12, an implantable pulse generator (IPG) 14, anexternal remote controller RC 16, a clinician's programmer (CP) 18, anExternal Trial Stimulator (ETS) 20, and an external charger 22.

The IPG 14 is physically connected via one or more percutaneous leadextensions 24 to the stimulation leads 12, which carry a plurality ofelectrodes 26 arranged in an array. In the illustrated embodiment, thestimulation leads 12 are percutaneous leads, and to this end, theelectrodes 26 may be arranged in-line along the stimulation leads 12. Inalternative embodiments, the electrodes 26 may be arranged in atwo-dimensional pattern on a single paddle lead (e.g., if cortical brainstimulation is needed). As will be described in further detail below,the IPG 14 includes pulse generation circuitry that delivers electricalstimulation energy in the form of a pulsed electrical waveform (i.e., atemporal series of electrical pulses) to the electrode array 26 inaccordance with a set of stimulation parameters.

The ETS 20 may also be physically connected via the percutaneous leadextensions 28 and external cable 30 to the stimulation leads 12. The ETS20, which has similar pulse generation circuitry as the IPG 14, alsodelivers electrical stimulation energy in the form of a pulse electricalwaveform to the electrode array 26 accordance with a set of stimulationparameters. The major difference between the ETS 20 and the IPG 14 isthat the ETS 20 is a non-implantable device that is used on a trialbasis after the stimulation leads 12 have been implanted and prior toimplantation of the IPG 14, to test the responsiveness of thestimulation that is to be provided.

The RC 16 may be used to telemetrically control the ETS 20 via abi-directional RF communications link 32. Once the IPG 14 andstimulation leads 12 are implanted, the RC 16 may be used totelemetrically control the IPG 14 via a bi-directional RF communicationslink 34. Such control allows the IPG 14 to be turned on or off and to beprogrammed with different stimulation parameter sets. The IPG 14 mayalso be operated to modify the programmed stimulation parameters toactively control the characteristics of the electrical stimulationenergy output by the IPG 14. As will be described in further detailbelow, the CP 18 provides clinician detailed stimulation parameters forprogramming the IPG 14 and ETS 20 in the operating room and in follow-upsessions.

The CP 18 may perform this function by indirectly communicating with theIPG 14 or ETS 20, through the RC 16, via an IR communications link 36.Alternatively, the CP 18 may directly communicate with the IPG 14 or ETS20 via an RF communications link (not shown). The clinician detailedstimulation parameters provided by the CP 18 are also used to programthe RC 16, so that the stimulation parameters can be subsequentlymodified by operation of the RC 16 in a stand-alone mode (i.e., withoutthe assistance of the CP 18).

The external charger 22 is a portable device used to transcutaneouslycharge the IPG 14 via an inductive link 38. For purposes of brevity, thedetails of the external charger 22 will not be described herein. Oncethe IPG 14 has been programmed, and its power source has been charged bythe external charger 22 or otherwise replenished, the IPG 14 mayfunction as programmed without the RC 16 or CP 18 being present.

For purposes of brevity, the details of the RC 16, CP 18, ETS 20, andexternal charger 22 will not be described herein. Details of exemplaryembodiments of these devices are disclosed in U.S. Pat. No. 6,895,280,which is expressly incorporated herein by reference.

Referring now to FIG. 6, the features of the stimulation leads 12 andthe IPG 14 will be briefly described. One of the stimulation leads 12(1)has eight electrodes 26 (labeled E1-E8), and the other stimulation lead12(2) has eight electrodes 26 (labeled E9-E16). The actual number andshape of leads and electrodes will, of course, vary according to theintended application. The IPG 14 comprises an outer case 40 for housingthe electronic and other components (described in further detail below),and a connector 42 to which the proximal ends of the stimulation leads12 mates in a manner that electrically couples the electrodes 26 to theelectronics within the outer case 40. The outer case 40 is composed ofan electrically conductive, biocompatible material, such as titanium,and forms a hermetically sealed compartment wherein the internalelectronics are protected from the body tissue and fluids. In somecases, the outer case 40 may serve as an electrode.

The IPG 14 includes a battery and pulse generation circuitry thatdelivers the electrical stimulation energy in the form of a pulsedelectrical waveform to the electrode array 26 in accordance with a setof stimulation parameters programmed into the IPG 14. Such stimulationparameters may comprise electrode combinations, which define theelectrodes that are activated as anodes (positive), cathodes (negative),and turned off (zero), percentage of stimulation energy assigned to eachelectrode (fractionalized electrode configurations), and electricalpulse parameters, which define the pulse amplitude (measured inmilliamps or volts depending on whether the IPG 14 supplies constantcurrent or constant voltage to the electrode array 26), pulse duration(measured in microseconds), pulse rate (measured in pulses per second),and burst rate (measured as the stimulation on duration X andstimulation off duration Y).

Electrical stimulation will occur between two (or more) activatedelectrodes, one of which may be the IPG case. Simulation energy may betransmitted to the tissue in a monopolar or multipolar (e.g., bipolar,tripolar, etc.) fashion. Monopolar stimulation occurs when a selectedone of the lead electrodes 26 is activated along with the case of theIPG 14, so that stimulation energy is transmitted between the selectedelectrode 26 and case. Bipolar stimulation occurs when two of the leadelectrodes 26 are activated as anode and cathode, so that stimulationenergy is transmitted between the selected electrodes 26. For example,electrode E3 on the first lead 12(1) may be activated as an anode at thesame time that electrode E11 on the second lead 12(1) is activated as acathode. Tripolar stimulation occurs when three of the lead electrodes26 are activated, two as anodes and the remaining one as a cathode, ortwo as cathodes and the remaining one as an anode. For example,electrodes E4 and E5 on the first lead 12 may be activated as anodes atthe same time that electrode E12 on the second lead 12 is activated as acathode.

The stimulation energy may be delivered between electrodes as monophasicelectrical energy or multiphasic electrical energy. Monophasicelectrical energy includes a series of pulses that are either allpositive (anodic) or all negative (cathodic). Multiphasic electricalenergy includes a series of pulses that alternate between positive andnegative. For example, multiphasic electrical energy may include aseries of biphasic pulses, with each biphasic pulse including a cathodic(negative) stimulation pulse and an anodic (positive) recharge pulsethat is generated after the stimulation pulse to prevent direct currentcharge transfer through the tissue, thereby avoiding electrodedegradation and cell trauma. That is, charge is conveyed through theelectrode-tissue interface via current at an electrode during astimulation period (the length of the stimulation pulse), and thenpulled back off the electrode-tissue interface via an oppositelypolarized current at the same electrode during a recharge period (thelength of the recharge pulse). The recharge pulse may be active, inwhich case, the electrical current is actively conveyed through theelectrode via current or voltage sources, or the recharge pulse may bepassive, in which case, the electrical current may be passively conveyedthrough the electrode via redistribution of the charge flowing fromcoupling capacitances present in the circuit.

As will be discussed in further detail below, the IPG 14 may beprogrammed by the CP 18 (or alternatively the RC 16) to generate fourpulsed electrical waveforms over four respective timing channels toprovide treatment to the patient in which the IPG 14 is implanted. Theelectrode combinations assigned to the respective timing channels willtypically be those that result in the treatment of four differentregions in the patient. Significantly, the IPG 14 allows overlap betweenthe electrical pulses generated in the respective timing channels.

Referring to FIG. 7, one example of using four timing channels tosimultaneously deliver pulsed electrical waveforms to groups of theelectrodes E1-E16, including the case electrode, will now be described.The horizontal axis is time, divided into increments of 1 millisecond(ms), while the vertical axis represents the amplitude of a currentpulse, if any applied to one of the sixteen electrodes and caseelectrode. Although, for purposes of simplicity, the pulsed electricalwaveforms are illustrated as being monophasic in nature, it should beappreciated that the pulsed electrical waveforms may be multiphasic innature.

At time t=0, channel 1 is set to generate and supply a current pulsehaving a pulse amplitude of 4 (milliamps) (mA), a pulse duration of 300microseconds (ps), and a pulse frequency of 200 pulses per second (pps)between electrode E1 (which appears as a 4 mA anodic (positive) pulse)and E3 (which appears as a −4 mA cathodic (negative) pulse). At timet=1, channel 2 is set to generate and supply a current pulse having apulse amplitude of 4 mA, a pulse duration of 300 ps, and a pulsefrequency of 250 pps between electrode E14 (+4 mA) and E13 (−4 mA). Attime t=2, channel 3 is set to generate and supply a current pulse havinga pulse amplitude 6 mA, a pulse duration of 300 μs, and a pulsefrequency of 200 pps between electrode E8 (+6 mA) and electrodes E6 andE7 (−4 mA and -2 mA, respectively). At t=3, channel 4 is set to generateand supply a current pulse having a pulse amplitude of 5 mA, a pulseduration of 400 μs, and a pulse frequency of 60 pps between electrodesE10 (+5 mA) and electrode E8 (−5 mA). At t=4, channel 1 is again set togenerate and supply the current pulse between electrodes E1 and E3, andchannel 2 is again set to generate and supply the current pulse betweenelectrodes E14 and E13. Notably, although the pulsed electricalwaveforms illustrated in FIG. 7 are monophasic in nature, the pulsedelectrical waveforms delivered during a timing channel can bemultiphasic in nature.

As shown in FIG. 8, the stimulation leads 12 are introduced through aburr hole 46 formed in the cranium 48 of a patient 44, and introducedinto the parenchyma of the brain 49 of the patient 44 in a conventionalmanner, such that the electrodes 26 are adjacent a target tissue regionwhose electrical activity is the source of the dysfunction (e.g., theventrolateral thalamus, internal segment of globus pallidus, substantianigra pars reticulate, subthalamic nucleus, or external segment ofglobus pallidus). Thus, stimulation energy can be conveyed from theelectrodes 26 to the target tissue region to change the status of thedysfunction. Due to the lack of space near the location where thestimulation leads 12 exit the burr hole 46, the IPG 14 is generallyimplanted in a surgically-made pocket either in the abdomen or above thebuttocks. The IPG 14 may, of course, also be implanted in otherlocations of the patient's body. The lead extension(s) 24 facilitateslocating the IPG 14 away from the exit point of the electrode leads 12.

Turning next to FIG. 9, the main internal components of the IPG 14 willnow be described. The IPG 14 includes stimulation output circuitry 50configured for generating electrical stimulation energy in accordancewith a defined pulsed waveform having a specified pulse amplitude, pulserate, pulse width, pulse shape, and burst rate under control of controlcircuitry 51 over data bus 54. The control circuitry 51 includes controllogic 52, which controls the electrodes to be activated, polarity of theactive electrodes, and amplitude of the current at the activeelectrodes, and timer logic 56, which controls the pulse frequency andpulse width of the pulsed electrical waveform. The stimulation energygenerated by the stimulation output circuitry 50 is output viacapacitors C1-016 to electrical terminals 55 corresponding to the leadelectrodes 26, as well as the case electrode 40. As will be discussed infurther detail below, the stimulation output circuitry 50 comprisesanodic current source circuitry and cathodic current source circuitry,each of which includes at least one non-dedicated source (i.e., a sourcethat can be temporally switched between selected ones of the leadelectrodes 26 and case electrode 40).

Any of the N electrodes may be assigned to up to k possible groups or“channels.” In one embodiment, k may equal four. The channel identifieswhich electrodes are selected to simultaneously source or sink currentto create an electric field in the tissue to be stimulated. Amplitudesand polarities of electrodes on a channel may vary, e.g., as controlledby the CP 18. External programming software in the CP 18 is typicallyused to set stimulation parameters including electrode polarity,amplitude, pulse rate and pulse duration for the electrodes of a givenchannel, among other possible programmable features.

The N programmable electrodes can be programmed to have a positive(sourcing current), negative (sinking current), or off (no current)polarity in any of the k channels. Moreover, each of the N electrodescan operate in a multipolar (e.g., bipolar) mode, e.g., where two ormore electrode contacts are grouped to source/sink current at the sametime. Alternatively, each of the N electrodes can operate in a monopolarmode where, e.g., the electrode contacts associated with a channel areconfigured as cathodes (negative), and the case electrode (i.e., the IPGcase) is configured as an anode (positive).

Further, the amplitude of the current pulse being sourced or sunk to orfrom a given electrode may be programmed to one of several discretecurrent levels, e.g., between 0 to 10 mA in steps of 0.1 mA. Also, thepulse duration of the current pulses is preferably adjustable inconvenient increments, e.g., from 0 to 1 milliseconds (ms) in incrementsof 10 microseconds (ps). Similarly, the pulse rate is preferablyadjustable within acceptable limits, e.g., from 0 to 1000 pulses persecond (pps). Other programmable features can include slow start/endramping, burst stimulation cycling (on for X time, off for Y time),interphase, and open or closed loop sensing modes.

Significantly, as will be described in further detail below, the controllogic 52 and timer logic 56 control the stimulation output circuitry 50in such a manner as to allow overlap between pulses in the channelsdespite the fact that a current source of the same polarity (eitheranodic or cathodic) is shared by the electrodes 26.

The IPG 14 further comprises monitoring circuitry 58 for monitoring thestatus of various nodes or other points 60 throughout the IPG 14, e.g.,power supply voltages, temperature, battery voltage, and the like. TheIPG 14 further comprises processing circuitry in the form of amicrocontroller (μC) 62 that controls the control logic over data bus64, and obtains status data from the monitoring circuitry 58 via databus 66. The IPG 14 additionally controls the timer logic 56. The IPG 14further comprises memory 68 and oscillator and clock circuitry 70coupled to the microcontroller 62. The microcontroller 62, incombination with the memory 68 and oscillator and clock circuit 70, thuscomprise a microprocessor system that carries out a program function inaccordance with a suitable program stored in the memory 68.Alternatively, for some applications, the function provided by themicroprocessor system may be carried out by a suitable state machine.

Thus, the microcontroller 62 generates the necessary control and statussignals, which allow the microcontroller 62 to control the operation ofthe IPG 14 in accordance with a selected operating program andstimulation parameters. In controlling the operation of the IPG 14, themicrocontroller 62 is able to individually generate a train of stimuluspulses at the electrodes 26 using the stimulation output circuitry 50,in combination with the control logic 52 and timer logic 56, therebyallowing each electrode 26 to be paired or grouped with other electrodes26, including the monopolar case electrode. In accordance withstimulation parameters stored within the memory 68, the microcontroller62 may control the polarity, amplitude, rate, pulse duration and channelthrough which the current stimulus pulses are provided. Themicrocontroller 62 also facilitates the storage of electrical parameterdata (or other parameter data) measured by the monitoring circuitry 58within memory 68, and also provides any computational capability neededto analyze the raw electrical parameter data obtained from themonitoring circuitry 58 and compute numerical values from such rawelectrical parameter data.

The IPG 14 further comprises an alternating current (AC) receiving coil72 for receiving programming data (e.g., the operating program and/orstimulation parameters) from the RC 16 (shown in FIG. 5) in anappropriate modulated carrier signal, and charging and forward telemetrycircuitry 74 for demodulating the carrier signal it receives through theAC receiving coil 72 to recover the programming data, which programmingdata is then stored within the memory 68, or within other memoryelements (not shown) distributed throughout the IPG 14.

The IPG 14 further comprises back telemetry circuitry 76 and analternating current (AC) transmission coil 78 for sending informationaldata sensed through the monitoring circuitry 58 to the CP 18. The backtelemetry features of the IPG 14 also allow its status to be checked.For example, when the CP 18 initiates a programming session with the IPG14, the capacity of the battery is telemetered, so that the externalprogrammer can calculate the estimated time to recharge. Any changesmade to the current stimulus parameters are confirmed through backtelemetry, thereby assuring that such changes have been correctlyreceived and implemented within the implant system. Moreover, uponinterrogation by the CP 18, all programmable settings stored within theIPG 14 may be uploaded to the CP 18.

The IPG 14 further comprises a rechargeable power source 80 and powercircuits 82 for providing the operating power to the IPG 14. Therechargeable power source 80 may, e.g., comprise a lithium-ion orlithium-ion polymer battery. The rechargeable battery 80 provides anunregulated voltage to the power circuits 82. The power circuits 82, inturn, generate the various voltages 84, some of which are regulated andsome of which are not, as needed by the various circuits located withinthe IPG 14. The rechargeable power source 80 is recharged usingrectified AC power (or DC power converted from AC power through othermeans, e.g., efficient AC-to-DC converter circuits, also known as“inverter circuits”) received by the AC receiving coil 72. To rechargethe power source 80, an external charger (not shown), which generatesthe AC magnetic field, is placed against, or otherwise adjacent, to thepatient's skin over the implanted IPG 14. The AC magnetic field emittedby the external charger induces AC currents in the AC receiving coil 72.The charging and forward telemetry circuitry 74 rectifies the AC currentto produce DC current, which is used to charge the power source 80.While the AC receiving coil 72 is described as being used for bothwirelessly receiving communications (e.g., programming and control data)and charging energy from the external device, it should be appreciatedthat the AC receiving coil 72 can be arranged as a dedicated chargingcoil, while another coil, such as coil 78, can be used forbi-directional telemetry.

It should be noted that the diagram of FIG. 9 is functional only, and isnot intended to be limiting. Those of skill in the art, given thedescriptions presented herein, should be able to readily fashionnumerous types of IPG circuits, or equivalent circuits, that carry outthe functions indicated and described, which functions include not onlyproducing a stimulus current or voltage on selected groups ofelectrodes, but also the ability to measure electrical parameter data atan activated or non-activated electrode.

Referring now to FIG. 10, the stimulation output circuitry 50 will nowbe described in further detail. The stimulation output circuitry 50employs programmable anodic current source circuitry 102, andprogrammable cathodic current source circuitry (also known as “currentsink circuitry”) 104. The cathodic source circuitry 104 is similar indesign and function the anodic source circuitry 102, although differingin polarity. For simplicity and to avoid redundancy, the components ofthe anodic current source circuitry 102 and the cathodic current sourcecircuitry 104 are generically discussed below.

Each of the anodic and cathodic current source circuitries 102, 104 isdivided into two parts: a coarse current source portion 106 and a finecurrent source portion 108, each of which fractionalizes the currentoutput by the respective current source circuitries 102, 104. As itsname suggests, the coarse current source portion 106 allows a coarseamount of current to be provided to a particular electrode. In otherwords, the amount of current that can be programmed to be sourced orsunk at a particular electrode can be adjusted in relatively largeincrements (e.g., 5%). By contrast, the amount of current that can beprogrammed to be sourced or sunk at a particular electrode by the finecurrent source portion 108 can be adjusted in relatively smallincrements (e.g., 1%).

Each of the anodic and cathodic current source circuitries 102, 104 alsocomprises a master reference digital-to-analog converter (DAC) 110,which generates a variable reference current I_(ref) that is supplied tothe coarse current source portion 106 and fine current source portion108 of the respective current source. The master DAC 110 of the anodiccurrent source circuitry 102 is referred to as a PDAC, and the masterDAC 110 of the cathodic current source circuitry 104 is referred to asan NDAC, reflecting the fact that transistors used in anodic currentsources are typically formed of P-type transistors that are biased to ahigh voltage (V+), whereas transistors used in cathodic current sourcesare typically formed of N-type transistors that are biased to a lowvoltage (V−).

Each master DAC 110 can comprise any structure known in the art forprogramming the amplification of current on the basis of a digitalcontrol signal. In the illustrated embodiment, each master DAC 110includes eight weighted banks of current sources (not shown), and thedigital control signal comprises eight bits that are respectivelyinputted along eight lines to the weighted banks of the master DAC 110,such that the master DAC 110 can be programmed to output 2 ⁸=256different values for the reference current I_(ref). In this way, thecurrents ultimately supplied to the coarse current source portion 106and fine current source portion 108 can be further (and globally) variedby adjusting the gain of the master DAC 110.

The coarse current source portion 106 does not involve dedicating orhard-wiring source circuitry to each electrode. Instead, the coarsecurrent source portion 106 is shared or distributed amongst the variouselectrodes via an L number of coarse current branches 114, each of whichincludes a current mirror 116 and an associated switch bank 118. In theexemplary case, L=19. In the illustrated embodiment, each of the currentmirrors 116 outputs the same current amplitude, and in particular, ascaled version of the reference current I_(ref), e.g., 5I_(ref). Inalternative embodiments, the current output by the current mirrors 116can be independently varied.

Notably, the current mirrors 116 are not individually adjustable in andof themselves (in contrast to the master DAC 110). Rather, the currentmirrors 116 supply matched currents to the switch banks 118, withselection or not of a particular current mirror's 116 current occurringin its given switch bank 118. Each of the switch banks 118 contains an Nnumber switches (not shown), which corresponds to the number ofelectrodes. In the exemplary case, N=17 (sixteen lead electrodes 26 andthe case electrode). Thus, each switch bank 118 is capable of routingthe current between its current mirror 116 and any of the leadelectrodes E1-E16 or case electrode (not shown in FIG. 10) in responseto a digital control signal. In the case of the anodic current sourcecircuitry 102, each switch routes the current from its current mirror toany of the electrodes, and in the case of the cathodic current sourcecircuitry 104, each switch routes the current from any of the electrodesto its current mirror.

In the illustrated embodiment, the digital control signal input intoeach switch bank 118 comprises seventeen bits that are respectivelyinput along seventeen respective lines to the switches of the switchbank 118, with only one of the seventeen bits being high, therebydesignating the specific switch in the respective switch bank 118 to beclosed, and thus, the corresponding electrode that receives the currentfrom the respective switch bank 118 (in the case of the anodic currentsource circuitry 102) or delivers the current to the respective switchbank 118 (in the case of the cathodic current source circuitry 104).Thus, only one switch per coarse current branch can be closed at onetime.

It can be appreciated from this, that multiple switch banks 118 can worktogether to produce a current at a given electrode. For example, whenoperating the anodic current source circuitry 102, and assuming thateach current mirror 116 supplies a current equal to 5I_(ref) to itsrespective switch bank 118, the coarse current source portion 106 cansupply a maximum current of L*I_(ref)=95I_(ref). If a current of50I_(ref) was desired at electrode E2, the corresponding switches of anyten of the switch banks 118 can be closed, e.g., the first ten switchbanks 118(1)-(10) or the last ten switch banks 118(10)-(19). Similarly,current can be supplied to multiple electrodes at the same time. Forexample, suppose that 50I_(ref) is desired at electrode E2; 10I_(ref) isdesired at electrode E5; and 15I_(ref) is desired at electrode E8. Thiscould be achieved by closing the switches corresponding to electrode E2(i.e., switches of ten of the switch banks (e.g., 118(1)-118(10));closing the switches corresponding to electrode E5 of two of the switchbanks (e.g., 118(11)-118(12)); and closing the switches corresponding toelectrode E8 of three of the switch banks (e.g., 118(13)-118(15)).

Because each of the coarse current branches 110 outputs a current equalto 5I_(ref), the minimum resolution of the coarse current source portion106 is 5I_(ref). Accordingly, the fine current source portion 108additionally provides the ability to make fine adjustments to thecurrent at the electrodes. Unlike the coarse current source portion 106,the fine current source portion 108 is preferably hard-wired to each ofthe electrodes 26. To the end, the fine current source portion 108comprises a fine current DAC 122 hardwired to each of the leadelectrodes E1-E16 and case electrode (not shown in FIG. 10). The finecurrent DAC 122 may be similar in design and architecture to the masterDAC 122 used to set the reference current I_(ref). Again, the finecurrent DAC 110 of the anodic current source circuitry 102 is referredto as a PDAC, and the fine current DAC 110 of the cathodic currentsource circuitry 104 is referred to as an NDAC. Each of the fine currentDACs 122 receives the reference current I_(ref) and outputs a variablecurrent in increments of the reference current I_(ref) in response to adigital input.

In the exemplary embodiment, each of the DACs 122 has a number J of finecurrent stages that can be activated in response to a digital controlsignal to output a variable current in a defined range, in increments ofI_(ref), to its respective electrode. For example, if J=5, each of theDACs 122 may supply a current ranging from 0I_(ref) to 5I_(ref) to itsrespective electrode. Thus, the fine current source portion 108 has acurrent resolution (I_(ref)) that is smaller than the current resolution(5I_(ref)) of the coarse current source portion 106. In the illustratedembodiment, the digital control signal input into each DAC 122 five bitsthat are respectively input along five lines into the five currentstages of the respective DAC 122, such that the DAC 122 can beprogrammed to output 6 different scaled values ranging from 0I_(ref) to5I_(ref).

Because of this difference in resolution, both the coarse current sourceportion 106 and the fine current source portion 108 can be usedsimultaneously to set a particular current at a given electrode. Forexample, assuming that it is desired to source a current of 53I_(ref) toelectrode E2, any ten of the coarse current branches 110 can beactivated to deliver 50I_(ref) to or from electrode E2. The fine currentDAC 122 coupled to electrode E2 can be programmed to deliver anadditional 3I_(ref) to or from electrode E2, resulting in the desiredtotal current of 53I_(ref) at electrode E2.

As one skilled in the art will appreciate, it is a matter of designchoice as to how many coarse current stages L are used, and how manyfine current stages J are used, and these values may be subject tooptimization. However if it is assumed that J fine current stages areused, then the number of coarse current stages L is preferably equal to(100/J)−1. Thus, if the number of fine current stages J is equal to 5,the number of coarse current stages L will be equal to 19, therebyallowing the coarse current source portion 106 to deliver approximately95% of the current range to or from any electrode with a resolution ofapproximately 5%, and allowing the fine current source portion 108 todeliver approximately 5% of the remaining current to or from anyelectrode at the resolution of approximately 1%. Although it ispreferred to use the same reference current I_(ref) as the input to thecurrent mirrors 116 in the coarse current source portion 106 and theDACs 122 in the fine current source portion 108, different referencecurrents can be used by the respective coarse current source portion 106and fine current source portion 108. In this case, separate master DAC'smay be programmed to generate different reference currents that are ascalar of each other.

In an optional embodiment, the stimulation output circuitry 50 furthercomprises a bank of recovery switches (not shown) respectively coupledbetween the electrodes and ground. In this case, a digital controlsignal comprising seventeen bits can be respectively input to theswitches to selectively switch any of the electrodes to ground, therebypassively recovering charge at the selected electrode or electrodes.

Further disclosure discussing the details of the stimulation outputcircuitry 50 can be found in U.S. Patent Publication No. 2007/0100399,which is expressly incorporated herein by reference.

Referring now to FIG. 11, control circuitry 51 (briefly discussed abovewith respect to FIG. 9) will be described. In response to an input ofstimulation parameter sets from the microcontroller 62, the controlcircuitry 51 is configured for instructing the stimulation outputcircuitry 50 to generate and convey a plurality of pulsed electricalwaveforms between the electrodes in a plurality of timing channels(e.g., as shown in FIG. 7 described above), with each pulsed electricalwaveform having a pulse amplitude, pulse duration, and pulse frequency,as specified by the respective stimulation parameter set defined by themicrocontroller 62.

In the illustrated embodiment, up to four pulsed electrical waveformscan be respectively generated within four timing channels. However, itshould be appreciated that the number of timing channels may differ. Asdiscussed above, the stimulation output circuitry 50 utilizes currentsource circuitry of different polarities (the anodic current sourcecircuitry 102 and the cathodic current source circuitry 104) to generateeach of the pulsed electrical waveforms. Although the use of both anodiccurrent source circuitry 102 and cathodic current source circuitry 104maximizes control of both the anodic and cathodic current amplitudesassigned to the respective electrodes, it should be appreciated thatonly anodic current source circuitry or only cathodic current sourcecircuitry may be utilized. It should also be appreciated that althoughthe stimulation output circuitry 50 utilizes current source circuitryfor purposes of current steering, the stimulation output circuitry 50may alternatively utilize voltage source circuitry.

Significantly, irrespective of whether the stimulation output circuitry50 utilizes one or both of an anodic electrical source circuitry orcathodic electrical source circuitry (whether current or voltage), thecontrol circuitry 51 allows the pulsed electrical waveforms to beindependently generated within the respective timing channels withoutconcern that pulses between the timing channels may overlap, and thus,without the need to manipulate the pulsed electrical waveforms in amanner that would effectively change the pulse frequencies of the pulsedelectrical waveforms.

In particular, when the pulses of respective electrical waveformsgenerated by the stimulation output circuitry 50 will not temporallyoverlap each other, the control circuitry 51 instructs the stimulationoutput circuitry 50 to couple the anodic source circuitry 102 todifferent anodic sets of the electrodes and to couple the cathodicsource circuitry 104 to different cathodic sets of the electrodes in aconventional manner to generate the non-overlapping pulses of therespective pulsed electrical waveforms.

For example, referring back to FIG. 7, timing channel 1 is the onlychannel operating at time t=0, and timing channel 2 is the only channeloperating at time t=1, and therefore, the control circuitry 51 willinstruct the stimulation output circuitry 50 to couple the anodic sourcecircuitry 102 to different anodic electrode sets respectively duringtimes t=0 and t=1 (i.e., an anodic electrode set comprising electrode E3at t=0, and a different anodic electrode set comprising E14 at t=1), andto couple the cathodic source circuitry 104 to different cathodicelectrode sets respectively during times t=0 and t=1 (i.e., a cathodicelectrode set comprising electrode E1 at t=0, and a different cathodicelectrode set comprising E13 at t=1). In contrast, both timing channels1 and 2 are operated at time t=4, and therefore, the control circuitry51 will instruct the stimulation output circuitry 50 to couple theanodic source circuitry 102 to a union of the different anodic electrodesets during time t=4 (i.e., an anodic electrode set comprisingelectrodes E3 and E14), and to couple the cathodic source circuitry 104to a union of the different cathodic electrode sets during time t=4(i.e., a cathodic electrode set comprising electrodes E1 and E13).

To this end, the control circuitry 51 generally comprises a plurality ofcurrent steering registers 152 and a plurality of timing registers 154configured to be programmed by the microcontroller 62 in accordance witha respective plurality of stimulation parameter sets stored within themicrocontroller 62. The control circuitry 51 further comprises aplurality of timers 156, each of which includes an input 166 coupled toan output 168 of a respective one of the timing registers 154. Thecontrol circuitry 51 further comprises a plurality of AND gates 158,each of which includes a first input 170 coupled to an output 172 of arespective one of the current steering registers 152, and a second input174 coupled to an output 176 of a respective one of the timing timers154. Because there are four timing channels, and thus, four stimulationparameter sets, the control circuitry 51 comprises four correspondingcurrent steering registers 152, four corresponding timing registers 154,four corresponding timers 156, and four corresponding AND gates 158.Notably, although a single steering register and a single timingregister may be used for each timing channel, in actuality, eachregister defined herein may comprise several discrete registers. Forexample, each electrode or a subset of electrodes may be associated withits own register. For the purposes of this specification, the term“register” may be defined as a set of discrete registers, whether suchregister set includes only one discrete register or a multitude ofdiscrete registers.

For the corresponding pulsed electrical waveform to be transmitted inthe corresponding timing channel, each of the current steering registers152 stores information related to the electrodes to be activated, aswell as the amplitude and polarity of the current at the activeelectrodes, in accordance with the corresponding stimulation parameterset stored within the microcontroller 62. For example, each currentsteering register 152 stores a digital representation of the set ofelectrodes to be activated for the respective pulsed electrical waveformto be generated during the respective timing channel. In one embodiment,the stored digital representation of the active electrode set takes theform of a plurality of binary values indicative of the relative currentamplitude values to be assigned to the electrodes.

In the illustrated embodiment, each binary value defines the polarityand number of coarse current source branches and fine current sourcebranches of the stimulation output circuitry 50 assigned to therespective electrode. In the exemplary case, a zero binary valueindicates that the corresponding electrode is not to be activated,whereas a non-zero binary value indicates that the correspondingelectrode is to be activated at a relative current value defined by thebinary value (i.e., the number of coarse and fine current branchesassigned to the electrode). In the illustrated embodiment, a polarityvalue of “1” indicates that the electrode is anodic, whereas a polarityvalue of “0” indicates that the electrode is cathodic, although itshould be appreciated that a polarity binary value of “0” may indicatethat the electrode is anodic, and a polarity value “1” may indicate thatthe electrode is anodic.

Each current steering register 152 also stores a digital global currentvalue to be assigned to the active electrodes for the respective pulsedelectrical waveform to be generated during the respective timingchannel. In the illustrated embodiment, the digital global current valueis indicative of the reference current I_(ref) output by the master DAC110 of the anodic current source circuitry 102 and the cathodic currentsource circuitry 104, which is preferably the same.

With reference to FIGS. 12 a-12 d, an example of the binary valuesstored by each of the current steering registers 152 will be described.

As there shown, for Timing Channel 1 (FIG. 12 a), a coarse currentbinary value of 00100, a fine current binary value of 100, and apolarity binary value of 0 are assigned to electrode E1 (i.e., fourcoarse current branches and four fine current branches of the cathodiccurrent source circuitry 104 are to be coupled to electrode E1), and acoarse current binary value of 00100, a fine current binary value of100, and a polarity binary value of 1 are assigned to electrode E1(i.e., four coarse current branches and four fine current branches ofthe anodic current source circuitry 102 are to be coupled to electrodeE3).

For Timing Channel 2 (FIG. 12 b), a coarse current binary value of00100, a fine current binary value of 100, and a polarity binary valueof 0 are assigned to electrode E13 (i.e., four coarse current branchesand four fine current branches of the cathodic current source circuitry104 are to be coupled to electrode E13), and a coarse current binaryvalue of 00100, a fine current binary value of 100, and a polaritybinary value of 1 are assigned to electrode E14 (i.e., four coarsecurrent branches and four fine current branches of the anodic currentsource circuitry 102 are to be coupled to electrode E14).

For Timing Channel 3 (FIG. 12 c), a coarse current binary value of00100, a fine current binary value of 100, and a polarity binary valueof 0 are assigned to electrode E6 (i.e., four coarse current branchesand four fine current branches of the cathodic current source circuitry104 are to be coupled to electrode E6), a coarse current binary value of00010, a fine current binary value of 010, and a polarity binary valueof 0 are assigned to electrode E7 (i.e., two coarse current branches andtwo fine current branches of the cathodic current source circuitry 104are to be coupled to electrode E7), and a coarse current binary value of00111, a fine current binary value of 001, and a polarity binary valueof 1 are assigned to electrode E8 (i.e., seven coarse current branchesand one fine current branch of the anodic current source circuitry 102are to be coupled to electrode E8).

For Timing Channel 4 (FIG. 12 d), a coarse current binary value of00110, a fine current binary value of 000, and a polarity binary valueof 0 are assigned to electrode E8 (i.e., six coarse current branches andzero fine current branches of the cathodic current source circuitry 104are to be coupled to electrode E8), and a coarse current binary value of00110, a fine current binary value of 000, and a polarity binary valueof 1 are assigned to electrode E10 (i.e., six coarse current branchesand zero fine current branches of the anodic current source circuitry102 are to be coupled to electrode E10).

As can be seen in FIGS. 12 a-12 d, a coarse current binary value of00000 and a fine current binary value of 000 are assigned to allinactive electrodes. For all of the timing channels, the digital globalcurrent value is set to be 10000000, such that half of the total anodiccurrent and cathodic current is made available to the active electrodesby the master DAC's 110. Thus, the assignment of coarse current branchesand fine current branches defines the relative current amplitudes at theactive electrodes, while the setting of the master DAC's 110 ultimatelydefines the global current amplitude at the active electrodes.

For the corresponding pulsed electrical waveform to be transmitted inthe corresponding timing channel, each of the timing registers 154stores information related to the pulse frequency and pulse duration inaccordance with the corresponding stimulation parameter set storedwithin the microcontroller 62.

Referring back to FIG. 11, each of the timers 156 outputs a phaseenabling 178 signal in accordance with a pulse duration and pulsefrequency defined by the timing information stored in the respectivetiming registers 154. That is, the phase enabling signal 178 output byeach timer 154 will be high (i.e., logical “1”) for a time equal to theduration of the pulses to be generated and at a frequency equal to thefrequency of the pulses to be generated. Thus, it can be appreciatedthat, when the phase enabling signal 178 is high (i.e., logical “1”),the digital contents stored in the corresponding register 152 are gatedto an output 180 of the AND gate 158 for a period of time equal to thepulse duration and at a frequency equal to the pulse frequency definedin the corresponding timing register 154.

The control circuitry 51 further comprises an OR gate 160 having inputs182-188 coupled to the output 180 of each of the respective AND gates158. Thus, any digital contents of the respective registers 152(including the digital representations of the active electrode sets)that are gated to the respective outputs 180 of the AND gates 156 arecombined at an output 190 of the OR gate 160 in a bit-wise manner (e.g.,the first bit of a first set of gated digital contents is AND'd with thefirst bit of a second set of gated digital contents; the second bit ofthe first set of gated digital contents is AND'd with the second bit ofthe second set of gated digital contents, etc.) to create a union of thedigital contents (effectively creating a union of the active electrodessets in the respective timing channels). Notably, when the digitalcontents of only one of the registers 152 are currently gated to itsrespective AND gate 158 (i.e., no pulses will overlap), the union of thedigital contents at the output 190 of the OR gate 160 will simply be thegated digital contents of that one current steering register 152. Incontrast, when the digital contents of more than one of the currentsteering registers 152 are currently gated to their respective AND gates156 (i.e., the pulses overlap), the union of the digital contents at theoutput 190 of the OR gate 160 will be the union of the multiple gateddigital contents.

For example, referring back to FIG. 7, during time t=0, the union of thegated digital contents at the output 190 of the OR gate 160 will simplybe the digital contents gated from the first register 152 (correspondingto timing channel 1); that is, the coarse current branch values, finecurrent values, and polarity values associated with electrodes E1 andE3, and during time t=1, the union of the gated digital contents at theoutput 190 of the OR gate 160 will simply be the digital contents gatedfrom the second register 152 (corresponding to timing channel 2); thatis, the coarse current branch values, fine current values, and polarityvalues associated with electrodes E13 and E14. In contrast, during timet=4, the union of the gated digital contents at the output 190 of the ORgate 160 will be the union of the digital contents gated both from thefirst register 152 and the second register 152, as illustrated in FIG.13. Notably, because the global current values will presumably be thesame for all of the timing channels, the union of the global currentvalues will be identical to the global current value.

It should be noted that although only one AND gate 158 per timingchannel is illustrated, there may be multiple AND gates per timingchannel. For example, certain portions of the digital contents withinthe current steering timing registers may be respectively gated to theoutputs of the multiple AND gates 158 by the phase enabling signals 178.For example, for each timing channel, the global amplitude binary valuesmay be gated to the output of a AND gate, the coarse branch binaryvalues and polarity value may be gated to the output of another ANDgate, and the fine branch binary values may be gated to the output ofstill another AND gate.

By the same token, it should also be noted that although only one ORgate 160 is illustrated for all timing channels, there may be multipleOR gates for all timing channels. For example, an OR gate can beprovided for each portion of the digital contents within the currentsteering timing registers to be combined. For example, the globalamplitude binary values gated to the respective outputs of AND gates canbe combined at the output of an OR gate to create a union of the globalamplitude binary values, the coarse branch binary values and polarityvalue gated to the respective outputs of AND gates can be combined atthe output of another OR gate to create a union of the coarse branchbinary values and polarity values, and the fine branch binary valuesgated to the respective outputs of AND gates can be combined at theoutput of still another OR gate to create a union of the fine branchbinary values.

In any event, the OR gate 160 outputs the union of the global currentvalues, the union of the coarse branch binary values, and the union ofthe fine branch binary values.

The union of the global current values gated at the output 190 of the ORgate 160 is routed as a digital control signal, which includes 8 bits,along eight lines directly to each of the master DACs 110 (see FIG. 10).For example, with reference to FIG. 13, the binary control signal10000000 will be routed to each of the master DACs 110 to deliver halfof available current to or from the anodic and cathodic current sourcecircuitries 102, 104.

The control circuitry 51 further comprises a coarse branch decoder anddistribution circuit 162 that obtains the union of the coarse currentbranch values and polarity values gated at the output of the OR gate160, and sends a digital control signal to the corresponding anodic andcathodic switch banks 118. In the illustrated embodiment, the coarsebranch decoder and distribution circuit 162 receives 102 bits (17electrodes×6 bits (five coarse branch bits and one polarity bit)) fromthe output 190 of the OR gate 160, and outputs nineteen digital controlsignals, each of which comprises 17 bits, along 17 lines to therespective anodic switch banks 118 (see FIG. 10), and nineteen digitalcontrol signals, each of which comprises 17 bits, along 17 lines to therespective cathodic switch banks 118 (see FIG. 10).

For example, with reference to FIG. 13, the coarse branch decoder anddistribution circuit 162 will, in accordance with the coarse currentbranch binary value “00100” and polarity value “0” associated withelectrode E1, make line 1 high and the remaining lines low for fourswitch banks 118 of the cathodic current source circuitry 104 to delivercurrent from the electrode E1 to four of the coarse current branches 114(20% of the cathodic current); in accordance with the coarse currentbranch binary value “00100” and polarity value “1” associated withelectrode E3, make line 3 high and the remaining lines low for fourswitch banks 118 of the anodic current source circuitry 102 to delivercurrent from four of the coarse current branches 114 to electrode E3(20% of the anodic current); in accordance with the coarse currentbranch binary value “00100” and polarity value “0” associated withelectrode E13, make line 13 high and the remaining lines low for fourswitch banks 118 of the anodic current source circuitry 102 to delivercurrent from four of the coarse current branches 114 to electrode E13(20% of the anodic current); in accordance with the coarse currentbranch binary value “00100” and polarity value “1” associated withelectrode E14, make line 14 high and the remaining lines low for fourswitch banks 118 of the cathodic current source circuitry 104 to delivercurrent from electrode E14 to four of the coarse current branches 114(20% of the cathodic current).

The control circuitry 51 further comprises a fine branch decoder circuit164 that obtains the union of the fine current branch values andpolarity values gated at the output of the OR gate 160, and sends adigital control signal to the corresponding anodic and cathodic finebranch DACs 122. In the illustrated embodiment, the fine branch decodercircuit 164 receives 68 bits (17 electrodes×4 bits (three fine branchbits and one polarity bit)) from the output 190 of the OR gate 160, andoutputs seventeen digital control signals, each of which comprises 5bits, to the fine PDACs 122 (see FIG. 10), and seventeen digital controlsignals, each of which comprises 5 bits, to the fine NDACs 122 (see FIG.10).

For example, with reference to FIG. 13, the fine branch decoder circuit164 will, in accordance with the fine current branch binary value “100”and polarity value “0” associated with electrode E1, make lines 1-4 highand line 5 low for the fine NDAC 122 associated with electrode E1 todeliver current from electrode E1 to four of the fine current branches(4% of the cathodic current); in accordance with the fine current branchbinary value “100” and polarity value “1” associated with electrode E3,make lines 1-4 high and line 5 low for the fine PDAC 122 associated withelectrode E3 to deliver current from four of the fine current branchesto electrode E3 (4% of the cathodic current); in accordance with thefine current branch binary value “100” and polarity value “0” associatedwith electrode E13, make lines 1-4 high and line 5 low for the fine NDAC122 associated with electrode E13 to deliver current from four of thefine current branches to electrode E13 (4% of the cathodic current); andin accordance with the fine current branch binary value “100” andpolarity value “1” associated with electrode E14, make lines 1-4 highand line 5 low for the fine PDAC 122 associated with electrode E14 todeliver current from electrode E14 to four of the fine current branches(4% of the cathodic current).

Notably, for each current source at any given time, each of the timingchannels can use any number of the coarse current branches as long asthe total number does not exceed L (i.e., the total number of coarsecurrent branches for each current source). Thus, as long as number ofcoarse current branches required by the timing channels at any giventime does not exceed the total number of coarse current branchesavailable, one timing channel will not affect another timing channel.Thus, because the timing channels can be operated to simultaneouslydeliver pulses, the pulsed electrical waveforms can have independentfrequencies.

It should also be noted that, when passive recharge pulses are utilized,the use of the currently activated electrodes in any particular timingchannel can be stored in a charge recovery module (not shown) associatedwith the respective timing channel. The timer associated with the timingchannel can enable passive recovery in the activated electrodes byenabling passive recovery switches (not shown) coupled between theseelectrodes and ground. During passive recovery, passive recovery signalsfor the electrodes can be gated on through an AND gate (not shown) bythe timer and the output combined with the passive recovery signals ofother timing channels through an OR gate (not shown). The passiverecovery signals at the output of the OR gate can then be sent to therecovery switches. When an active pulse coincides with a charge recoverypulse, the passive recovery signals may be automatically inhibited(gated off) until the end of the active pulse. At the end of the activepulse, recovery resumes until the end of the passive recovery phase.

Although particular embodiments of the present inventions have beenshown and described, it will be understood that it is not intended tolimit the present inventions to the preferred embodiments, and it willbe obvious to those skilled in the art that various changes andmodifications may be made without departing from the spirit and scope ofthe present inventions. Thus, the present inventions are intended tocover alternatives, modifications, and equivalents, which may beincluded within the spirit and scope of the present inventions asdefined by the claims.

1. A multi-channel neurostimulation system, comprising: a plurality ofelectrical terminals configured for being respectively coupled to aplurality of electrodes; stimulation output circuitry includingelectrical source circuitry of the same polarity configured forgenerating a plurality of pulsed electrical waveforms in a plurality oftiming channels; and control circuitry configured for instructing thestimulation output circuitry to serially couple the electrical sourcecircuitry to different sets of the electrodes when pulses of therespective pulsed electrical waveforms do not temporally overlap eachother, and for instructing the stimulation output circuitry to couplethe electrical source circuitry to a union of the different electrodesets when pulses of the respective pulsed electrical waveformstemporally overlap each other.
 2. The neurostimulation system of claim1, wherein the pulsed electrical waveforms are defined by a respectiveplurality of stimulation parameter sets; wherein the control circuitryis configured for obtaining a digital representation of an electrode setfrom each of the stimulation parameter sets, combining the digitalrepresentations together to create a union of the digitalrepresentations, and outputting the digital representation union to thestimulation circuitry; and wherein the stimulation output circuitry isconfigured for coupling the electrical source circuitry to the union ofthe different electrode sets in accordance with the digitalrepresentation union.
 3. The neurostimulation system of claim 2, whereineach of the digital representations comprises a digital representationof current amplitude values for the respective electrode set, andwherein the control circuitry is configured for instructing thestimulation output circuitry to supply electrical current from theelectrical source circuitry to the different sets of the electrodes orthe union of the different electrode sets in accordance with the currentamplitude values.
 4. The neurostimulation system of claim 1, wherein thestimulation output circuitry includes at least one switch bank coupledbetween the electrical source circuitry and the electrical terminals. 5.The neurostimulation system of claim 1, wherein the electrical sourcecircuitry comprises current source circuitry.
 6. The neurostimulationsystem of claim 5, wherein the current source circuitry comprises aplurality of current branches, and wherein the control circuitry isconfigured for selecting a current magnitude for each electrode in theunion of the different electrode sets by assigning one or more of thecurrent branches to the respective electrode.
 7. The neurostimulationsystem of claim 1, wherein the control circuitry includes: a pluralityof registers configured for storing digital representations of theelectrode sets; a plurality of timers configuring for outputting phaseenabling signals in accordance with a pulse duration and pulse frequencyof the pulsed electrical waveforms; a plurality of AND gates, each ofwhich has an input coupled to an output of a respective one of theregisters and an input coupled to an output of a respective one of thetimers; and an OR gate having inputs coupled to respective outputs ofthe AND gates, and an output coupled to an input of the stimulationoutput circuitry.
 8. The neurostimulation system of claim 7, wherein thephase enabling signal output by each of the timers is high when thepulse of the respective pulsed electrical waveform is active.
 9. Theneurostimulation system of claim 1, wherein the pulsed electricalwaveforms have different frequencies.
 10. The neurostimulation system ofclaim 1, further comprising the plurality of electrodes.
 11. Theneurostimulation system of claim 1, further comprising a housingcontaining the plurality of electrical terminals, stimulation outputcircuitry, and control circuitry.
 12. A multi-channel neurostimulationsystem, comprising: a plurality of electrical terminals configured forbeing respectively coupled to a plurality of electrodes; a plurality ofregisters configured for storing a respective plurality of digitalrepresentations of different sets of the electrodes; a plurality oftimers configuring for outputting phase enabling signals in accordancewith a pulse duration and pulse frequency of a respective plurality ofpulsed electrical waveforms; a plurality of AND gates, each of which hasan input coupled to an output of a respective one of the registers andan input coupled to an output of a respective one of the timers; an ORgate having inputs coupled to respective outputs of the AND gates; andstimulation output circuitry having an input coupled to an output of theOR gate, the stimulation output circuitry including electrical sourcecircuitry of the same polarity programmable to selectively couple to theelectrodes via the electrical terminals based on the output of the ORgate.
 13. The neurostimulation system of claim 12, wherein the timersare operated independently of each other.
 14. The neurostimulationsystem of claim 12, wherein the electrical source circuitry comprisescurrent source circuitry.
 15. The neurostimulation system of claim 14,wherein each of the digital representations comprises a digitalrepresentation of current amplitude values for the respective electrodeset, and the current source circuitry is programmable to supply currentto the electrodes based on the output of the OR gate.
 16. Theneurostimulation system of claim 15, wherein the current sourcecircuitry comprises a plurality of current branches, the system furthercomprising a branch distribution circuit configured for assigning one ormore of the current branches to each of the electrodes based on theoutput of the OR gate.
 17. The neurostimulation system of claim 16,wherein each of the current branches comprises: a switch bank coupled tothe electrodes; and a decoder coupled to the respective switch bank,wherein the branch distribution circuit is configured for supplying adigital code to each of the decoders, the digital code defining one ofthe electrodes to be coupled to the electrical source circuitry via therespective switch bank.
 18. The neurostimulation system of claim 12,wherein the pulsed electrical waveforms have different frequencies. 19.The neurostimulation system of claim 12, further comprising theplurality of electrodes.
 20. The neurostimulation system of claim 12,further comprising a housing containing the plurality of electricalterminals, the plurality of registers, the plurality of timers, theplurality of AND gates, the OR gate, and the stimulation outputcircuitry.